Although a wide variety of aliphatic carboxylic acids of differing carbon numbers and structures are presently important articles of commerce, production of acetic acid is especially desirable. Important applications for this acid include the production of cellulose acetate and vinyl acetate. There are several commercially proven routes to acetic acid manufacture, including oxidation of ethylene via acetaldehyde, liquid-phase oxidation of saturated hydrocarbons, n-butane oxidation and methanol carbonylation. To the extent that methanol is currently produced from synthesis gas (a mixture of carbon monoxide and hydrogen), acetic acid via methanol carbonylation also effectively becomes a `syngas` chemical. Furthermore, since syngas may be generated from a variety of sources, including heavy oil residuals and coal stocks, this syngas route to acetic acid will likely become increasingly important. (See: "Trends in Petrochemical Technology" by A. M. Brownstein (1976), Chapters 4 and 5; and "Petrochemicals from Coal" by P. M. Spitz, Chemtech, May 1977, p. 295.
Carbonylation processes for the preparation of carboxylic acids from alcohols are well known in the art. These have been directed especially to the production of acetic acid by the carbonylation of methanol. In particular, a variety of soluble and supported forms of cobalt, nickel, iron, iridium and rhodium have been patented as catalysts for methanol carbonylation to acetic acid. In the case of carbonylation processes of the prior art, comprising processes employing metal carbonyls or modified metal carbonyls of cobalt, iron and nickel, each is characterized by the need for high partial pressures of carbon monoxide in order that the carbonyls remain stable under the 200.degree. C. temperatures normally employed. See: "Carbon Monoxide in Organic Synthesis" by J. Falbe (1976), Chapters II and III. Dicobalt octacarbonyl, for example, requires partial pressures of carbon monoxide in the 4,000 psi to 10,000 psi range. Furthermore, said cobalt, nickel and iron catalysts of the prior art generally display relatively poor selectivities to the desired carboxylic acids due to the substantial formation of undesirable by-products. Said by-products comprise substantial amounts of ethers, aldehydes, higher carboxylic acids, carbon dioxide, methane and water. See: N. Von Kutepow, et al., Chemie-Ing. Techn. 37,383 (1965).
A series of very active carbonylation catalysts have been patented. See for example: Belgium Pat. No. 713,296 (1968), U.S. Pat. No. 3,772,380 (1973) and U.S. Pat. No. 3,717,670 (1973), where the active constituents contain a rhodium or iridium component in combination with a halogen promoter. These catalyst combinations are characterized by being effective under relatively mild operating conditions and achieving high selectivity to desired acetic acid in the case of methanol carbonylation. However, both iridium and rhodium are rare, costly metals, and rhodium in particular is predicted to be in increasingly short supply due to expanded uses in petrochemical catalysis and in catalytic muffler applications. Furthermore, in recent reports, it is noted that much dimethylether is also formed during the rhodium-catalyzed carbonylation of methanol in pure methanol solvent. See: T. Matsumato et al, Bull, Chem. Soc. Japan, 50, 2337 (1977).
Roth et al. describes a homogeneous liquid phase catalyst which is capable of bringing about methanol carbonylation at 1 atm in 99% selectivity using a rhodium compound, an iodide promoter and a solvent. Although this system is very effective for producing acetic acid, the corrosive iodine promoter is still present and rhodium is expensive. See: Roth, J. F. et al Chem. Technol. 600, October (1971).
In a discussion of the art of carbonylation of methanol to acetic acid, Forster attempts to define various rhodium species present in the catalytic cycle when rhodium(III) halide in particular is charged to the reaction as the catalyst precursor. In his model he presents a pathway for the reaction which is consistent with the observed independence of the overall reaction rate of carbon monoxide pressure and methanol concentration. See: Forster, D. JACS 98 846 (1976).
An article in J. of Catalysis 47 269, (1977), enumerates the disadvantages of the best known methanol carbonylation reactions in that the reactors, separators and recycle loops must be constructed of expensive corrosion-resistant materials and that recovery of the catalyst and promoter from the reaction products requires several separators because of the relatively high volatility of the iodide compounds. In this article an attempt is made to identify a suitable promoter substitute for idodide. Promoters chosen were pentafluoro and pentachlorobenzenethiol; however, the effectiveness of these promoters is much less than with iodide. For example, the rate of methanol carbonylation is about 4% of the rate with methyl iodide at a comparable temperature.
U.K. Patent application GB No. 2007212A by Isshiki and Kijima discloses a method of producing carboxylic acids by reacting alcohols with carbon monoxide in the presence of elemental nickel or a nickel compound, an organic compound of a trivalent nitrogen-group element, and iodine or an iodine compound. The asserted advantage of this system is the use of milder conditions and an inexpensive catalyst, but iodine is still necessary.
In U.S. Pat. No. 3,856,856, Nozaki discloses the use of a platinum promoter in a cobalt-iodide catalyst system. Here, the yield of methyl acetate is generally higher than that of acetic acid.
In U.K. Patent application GB No. 2007658A, Isshiki and Kijima disclose a method for reacting an alcohol with carbon monoxide to produce an aliphatic carboxylic acid using a Group VIII compound and at least one iodine-containing compound as a promoter, optionally in the presence of a trivalent nitrogen-group element as an accelerating agent, and a solvent.
In U.S. Pat. No. 3,769,324, to Paulik et al. a process is disclosed for the preparation of aromatic carboxylic acids and esters in the presence of a catalyst system including a metal selected from the group consisting of iridium, osmium and ruthenium and a halogen component.
The various catalyst systems of the prior art all have distinct disadvantages. Either selectivities are not very high, extremely high pressures are necessary, expensive and increasingly hard to find metal catalysts are used, or, the most prevalent disadvantage is the need to use an iodide promoter which is corrosive and results in added expense in constructing and maintaining reactors, separators and recycle loops. Although the system devised by Roth et al. is very effective, the use of iodine is a major disadvantage.
Initial attempts to devise carbonylation catalyst systems which use promoters other than iodide have been unsuccessful or have resulted in systems with very poor conversion and selectivity.
As will be discussed, it is an object of this invention to design a catalyst system for carbonylation of methanol to acetic acid which uses mild conditions including mild pressures, does not use expensive rhodium, does not use a corrosive iodide promoter and which has selectivities high enough for commercial consideration.